1. Field of the Invention
The present invention relates to pyrene based compounds and materials and their use in organic light emitting devices. These pyrene based compounds and materials can be used as the emissive layer, electron transport layer, hole transport layer or one or more of such layers, although their use as a blue emissive layer is preferred. These compounds can also be used as a host or dopant material for one or more of such layers.
2. Description of the Related Art
Organic light emitting devices (OLEDS) typically comprise a layer of emissive material between an anode and a cathode. When a bias is applied across the electrodes, positive charges (holes) and negative charges (electrons) are respectively injected from the anode and cathode into the emissive layer. The holes and the electrons form excitons in the emissive layer to emit light.
Electrodes are chosen to facilitate charge injection. A transparent indium-tin-oxide (ITO) anode has a relatively high work function and is therefore suitable for use as a hole injection electrode, while low work function metals such as Al, Mg and Ca are suitable for injection of electrons.
To improve the power efficiency of an OLED, it is frequently desirable to enhance charge injection at the electrode interface. Hole transport layers and electron transport layers may be added adjacent to the respective electrodes to facilitate charge transfer. Depending upon whether hole transport or electron transport is favored, the light emissive layer may be located closer to the anode or the cathode. In some instances, the emissive layer is located within the hole transport or electron transport layer.
Improved performance can be obtained if blocking layers are provided to block against the injection of either holes or electrons from the adjoining layer and their subsequent escape from the device. Likewise, a modifying layer may be used to improve the contact with one or both of the electrodes, or to improve the interface between two other layers.
Some of these layers can be combined. For example, a double-layered structure is fabricated from a combined hole-injecting and transporting layer together with a combined electron-transporting and light-emitting layer. Likewise, a triple-layered structure is composed of a hole-injecting and transporting layer, a light-emitting layer, and an electron-injecting and transporting layer.
Hole transport layers may include triarylamnine-based materials, although many other hole transport materials are known. Likewise, an aluminum quinolinolate complex known as AlQ3 is a well known electron-transport material which has been used in OLEDs, although other electron transport materials are known.
Emissive materials having widely varied structures are known in the art and are generally selected based on color, brightness, efficiency and lifetime. These emissive materials may themselves also have electron transport or hole transport characteristics.
In addition, it is possible to form these layers from a “host” material doped with another material (the “guest” material) designed to achieve the desired effect of the layer (for example, to achieve a hole transport effect, an electron transport effect, or an emissive effect). In the case of an emissive guest-host system, the host must be able to transfer energy to the guest so that a maximum amount of energy contributes to emission by the guest rather than being absorbed by the host.
Fused aromatic ring compounds have been used in the layers of organic light emitting devices. Their advanced pi-delocalization system, high mobility and good photoluminescence are desired qualities for OLED application.
Pyrene is a fused aromatic ring compound. In solution, pyrene fluoresces purple blue, yet in solid state it fluoresces white. This white light is due to intermolecular aggregation.
Since blue emissive materials are desired, it has been considered to prevent aggregation in pyrene by attaching pyrene to a benzene ring at the 1, 3, and 5 position to make 1,3,5-tripyrene benzene (3TPB), as follows: 
This arrangement results in a good blue emissive material with a peak emission at 450 nm. However, closer investigation of 3TPB reveals that the compound still has a minor aggregation problem in its solid state, resulting in a shoulder emission at 482 nm and reduced blue color purity.
There continues to be a need for OLED materials exhibiting thermal stability, having bright, high purity luminescent emission, and for materials which contribute to greater luminescence per injected charge. There is particularly a need for OLED materials which provide a good blue emission.